Redox conditions in the atmosphere and shallow-marine environments during the first Huronian deglaciation: Insights from Os isotopes and redox-sensitive elements

- Sedimentary rocks deposited during the first Huronian deglaciation were studied.
- We found chondritic compositions of Os isotopes and enrichments of Re in the rocks.
- Oxidative weathering of Os was absent, but that of Re might have occurred.
- The atmospheric O2 levels would remain mildly oxidizing during the deglaciation.
- These results contrast with the cases of the second and third Huronian glaciations.

The Paleoproterozoic (2.5-2.0 Ga) is one of the most important periods in Earthʼs history, and was characterized by a rise in atmospheric oxygen levels and repeated (at least three) severe glaciations (the Huronian glaciations). In this study, we investigate redox conditions in the atmosphere and in shallow-marine environments immediately after the first Huronian glaciation based on the isotopic composition of Os, and the abundance of redox-sensitive elements (Os, Re, and Mo) in sedimentary rocks from the Huronian Supergroup, Canada. We found no significant authigenic enrichment of Os in the sedimentary rocks deposited during the first Huronian deglaciation. The initial isotopic composition of Os in the sediments was close to that of chondrite at the time of deposition (Os187/188Os=∼0.11). These results suggest that atmospheric O2 levels were insufficient to mobilize radiogenic Os through continental weathering (pO2<10−5-10−3 present atmospheric level (PAL)). In contrast, we found enrichment of Re in the sedimentary rocks, which suggests the occurrence of oxidative weathering of Re under mildly oxidizing conditions (>10−8-10−5 PAL). Despite the Re enrichment, low abundances of Mo imply possible non-sulfidic conditions in shallow-marine environments at the time of deposition. Together with the results of organic carbon and sulfur analyses, we suggest that atmospheric O2 remained at relatively low levels of around 10−8-10−5 PAL after the first Huronian deglaciation, which contrasts with proposed dramatic increases in O2 after the second and third Huronian deglaciations. These results imply that the second and third Huronian glaciations may have been global events, associated with climatic jumps from severe glaciations to super-greenhouse conditions and the subsequent blooming of photosynthetic cyanobacteria in the glacial aftermath.